The invention relates to a method of manufacturing a dispenser cathode comprising a barium compound for dispensing barium to an emissive surface of a porous cathode body substantially comprising a metal melting at a high temperature.
The invention also relates to a dispenser cathode manufactured by such a method and to an electron tube provided with such a cathode.
A characteristic feature of dispenser cathodes is that there is a functional separation between the electron emissive surface on the one hand and a store of emitter material on the other hand. The emitter material is present in the pores of the porous metal cathode body and is used for realizing a sufficiently low work function on the emissive surface.
A method of the type mentioned in the opening paragraph is described in U.S. Pat. No. 4,007,393. This patent describes how a cathode body with a porosity of approximately 20% is compressed from tungsten powder, sintered and impregnated with a mixture which comprises calcium oxide, aluminium oxide and scandium oxide in addition to barium oxide.
European Patent Specification No. 0,091,161, corresponding to U.S. Pat. No. 4,625,142, describes how sensitivity to and recovery from ion bombardment of such cathodes can be improved by forming the cathode body (notably the top layer) from a mixture of tungsten powder and scandium oxide powder which is compressed and sintered. To obtain a cathode body with a thin top layer (approximately 0.1 mm) which is as homogeneous as possible the compressing operation is generally performed in two steps. Firstly, the tungsten portion of the cathode body is slightly pre-compressed. Subsequently, the top layer powder is evenly distributed over a surface of the tungsten portion whereafter the definitive compressing operation is performed.